Epitaxial film forming method, sputtering apparatus, manufacturing method of semiconductor light-emitting element, semiconductor light-emitting element, and illumination device

ABSTRACT

The present invention provides an epitaxial film forming method for epitaxially growing a high-quality group III nitride semiconductor thin film on an α-Al 2 O 3  substrate by a sputtering method. In the epitaxial film forming method according to an embodiment of the present invention, when an epitaxial film of a group III nitride semiconductor thin film is to be formed on the α-Al 2 O 3  substrate arranged on a substrate holder provided with a heater electrode and a bias electrode of a sputtering apparatus, in a state where the α-Al 2 O 3  substrate is maintained at a predetermined temperature by the heater electrode, high-frequency power is applied to a target electrode and high-frequency bias power is applied to a bias electrode and at that time, the powers are applied so that frequency interference between the high-frequency power and the high-frequency bias power does not occur.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation application of International Application No. PCT/JP2011/007040, filed Dec. 16, 2011, which claims the benefit of Japanese Patent. Application No. 2010-2892.65, filed Dec. 27, 2010. The contents of the aforementioned applications are incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present invention relates to an epitaxial film forming method, a sputtering apparatus, a manufacturing method of a semiconductor light-emitting element, a semiconductor light-emitting element, and an illumination device and particularly to an epitaxial film forming method capable of forming a high-quality epitaxial film, a manufacturing method of a semiconductor light-emitting element using such epitaxial film, a sputtering apparatus, a semiconductor light-emitting element, and an illumination device.

BACKGROUND ART

Group III nitride semiconductors are compound semiconductor materials of an aluminum (Al) atom, a gallium (Ga) atom, and an indium (In) atom which are group IIIB elements (hereinafter referred to simply as III elements) and a nitrogen (N) atom which is a group VB element (hereinafter referred to simply as a group V element). That is, compound semiconductor materials obtained as aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN) and their mixed crystals (AlGaN, InGaN, InAlN, InGaAlN) are group III nitride semiconductors.

Elements using the group III nitride semiconductors include optical devices such as light-emitting diodes covering a wide wavelength region over far-ultraviolet/visible/near-infrared regions (LED: Light Emitting Diode), laser diode (LD), solar cell (PVSC: Photovoltaic Solar Cell), photodiode (PD) and the like and electronic devices such as high electron mobility transistor for high-frequency/high-output applications (HEMT: High Electron Mobility Transistor), metal-oxide-semiconductor field effect transistor (MOSFET)and the like.

In order to realize element applications as above, it is necessary to epitaxially grow a group III nitride semiconductor thin film on a single crystal substrate so as to obtain a high-quality single crystal film with few crystal defects (epitaxial film). However, since a single crystal substrate made of a group III nitride semiconductor is extremely expensive, it is not used except some applications, and a single crystal film is obtained mainly by hetero-epitaxial growth of sapphire (α-Al₂O₃), silicon carbide (SiC) on dissimilar substrates. Particularly, α-Al₂O₃ substrates are inexpensive and those with a large area and high quality can be obtained, and therefore almost all of LEDs using the group III nitride semiconductor thin film sold in the market use α-Al₂O₃ substrates.

For the epitaxial growth of such group III nitride

semiconductor thin film, a metal organic chemical vapor deposition (MOCVD) method by which high-productivity and high-quality epitaxial films can be obtained is used. However, the MOCVD method has problems that a production cost is high, particles can be easily generated and it is difficult to obtain high yield and the like.

On the other hand, a sputtering method has features that, a production cost can be suppressed low and particle generation probability is also low. Therefore, if at least a part of a film forming process of the group XII nitride semiconductor thin film can be replaced by the sputtering method, it is likely that at least a part of the above-described problems can be solved.

However, the group III nitride semiconductor thin film fabricated by using the sputtering method has a problem that a crystal quality tends to become poorer than those fabricated by using the MOCVD method. For example, crystalline of the group III nitride semiconductor thin film fabricated by using the sputtering method is disclosed in NPL 1, for example. In NPL 1, a GaN film with c-axis orientation is epitaxially grown on an α-Al₂O₃ (0001) substrate by using a high-frequency magnetron sputtering method. NPL 1 describes that in X-ray rocking curve (XRC) measurement of a GaN (0002) plane, its full width at half maximum (FWHM) is 35.1 arcmin (2106 arcsec), This value is an extremely large value as compared with a GaN film on the α-Al₂O₃ substrate currently sold in the market and indicates that mosaic expansion of tilt which will be described later is large and crystal quality is poor.

Here, concepts used as indexes indicating the crystal quality, that is, (1) mosaic expansion of tilt, (2) mosaic expansion of twist, and (3) polarity will be described in brief. The mosaic expansion or tilt in (1) indicates a degree of variation in crystal orientation in a substrate perpendicular direction, and the mosaic expansion of twist in (2) indicates a degree of variation in crystal orientation in a substrate in-plane direction. The polarity in (3) is a term meaning an orientation of a crystal, and in the case of c-axis orientation film, there are two types of growth modes, that is, +c polarity and −c polarity. The growth with the +c polarity corresponds to (0001) orientation and the growth with −c polarity corresponds to (000-1) orientation,

It is necessary for a single crystal with favorable crystalline that mosaic expansions of tilt and twist are small and also, polarity is biased to either of the +c polarity or the −c polarity. Particularly, since a group III nitride semiconductor thin film with favorable morphology and excellent crystalline can be easily obtained with the +c polarity, establishment of a process of obtaining group III nitride semiconductors with the +c polarity is in demand. On the other hand, many attempts have been made in order to obtain a good-quality group III nitride semiconductor thin film by the sputtering method (See PTLs 1 and 2).

PTL 1 discloses a method of realizing higher quality of group III nitride semiconductor thin films by applying plasma processing to a substrate before a group III nitride semiconductor thin film (AlN in PTL 1) is formed on an α-Al₂O₃ substrate by using the sputtering method or particularly a method of obtaining a group III nitride semiconductor thin film with extremely small mosaic expansion of tilt.

Moreover, PTL 2 discloses a manufacturing method of a group III nitride semiconductor (group III nitride compound semiconductor in PTL 2) light-emitting element in which a buffer layer (an intermediate layer in PTL 2) made of the group III nitride semiconductor (group III nitride compound in PTL 2) is formed on a substrate by the sputtering method and an n-type semiconductor layer provided with a base film, a light-emitting layer, and a p-type semiconductor layer are sequentially laminated on the buffer layer made of this group III nitride semiconductor.

PTL 2 describes that, as a procedure for forming the buffer layer made of the group III nitride semiconductor, a pre-treatment process of applying plasma processing to a substrate and a process of depositing the buffer layer made of the group III nitride semiconductor by sputtering method subsequently to the pre-treatment process. Moreover, in PTL 2, as a preferable mode of the substrate and the buffer layer made of the group III nitride semiconductor, an α-Al₂O₃ substrate and AlN are used, and as a deposition method of the n-type semiconductor layer provided with a base film, the light-emitting layer, and the p-type semiconductor layer, the MOCVD method is preferably used.

CITATION LIST Patent Literature

PTL 1: International Publication No. 2009/096270

PTL 21 Japanese Patent Application Laid-Open No. 2008-109084

Non Patent Literature

NPL 1: Y. Daigo, N. Mutsukura, “Synthesis of epitaxial GaN single-crystalline film by ultra high vacuum r.f. magnetron sputtering method”, Thin Solid Films 483 (2005) p38-43.

SUMMARY OF INVENTION

According to the already disclosed prior-art technologies (PTL 1 and PTL 2), a group III nitride semiconductor having small mosaic expansion of tilt or twist is obtained by the sputtering method. However, the prior-art technologies do not disclose a method of controlling polarity, and there is a serious problem in employing the sputtering method as a manufacturing process of a group III nitride semiconductor.

Actually, when an AlN film was formed on an α-Al₂O₃ substrate by the sputtering method by using the technologies disclosed in PTLs 1 and 2, an AlN film with small mosaic expansion of tilt or twist could be obtained but the +c polarity and the −c polarity were mixed with regard to the polarity. Moreover, when a GaN film was grown on the AlN film in which the +c polarity and the −c polarity were mixed by the MOCVD method, a high-quality GaN film could not be obtained. Moreover, a light-emitting element was fabricated by using the obtained GaN film, but favorable emission characteristics could not be obtained. Therefore, mixture of the +c polarity and the −c polarity is not reduced and a group III nitride semiconductor thin film with the +c polarity cannot be obtained only by the technologies disclosed in PTLs 1 and 2. That is, though the technologies disclosed in PTLs 1 and 2 are effective technologies since the mosaic expansion of tilt or twist can be made small, unification of polarity as much as possible is in demand in order to obtain a group III nitride semiconductor thin film with a higher quality.

The present invention is made in view of the above-described problems and has an object to provide an epitaxial film forming method capable of fabricating an epitaxial film with improved degree of unification of the +c polarity (improved (0001) orientation) and further to provide a manufacturing method of a semiconductor light-emitting element using this epitaxial film, a sputtering apparatus, a semiconductor light-emitting element manufactured by this manufacturing method, and an illumination device.

The inventors have obtained new founding as the result of keen examination that polarity of an epitaxial film can be controlled by high-frequency bias electric power applied to a bias electrode built in a substrate holder as will be described later and completed the present invention.

In order to achieve the above-described object, a first aspect of the present invention is an epitaxial film forming method that uses a sputtering apparatus having a target electrode on which a target can be arranged and a substrate holder on which a substrate can be arranged toward the target electrode and provided with a heater electrode and a bias electrode, and that epitaxially grows a group III nitride semiconductor thin film by a sputtering method on an α-Al₂O₃ substrate arranged on the substrate holder, the method comprising the steps of; arranging the α-Al₂O₃ substrate on the substrate holder; and forming an epitaxial film of the group III nitride semiconductor thin film on the α-Al₂O₃ substrate arranged on the substrate holder, wherein the step of forming an epitaxial film of the group III nitride semiconductor thin film maintains the α-Al₂O₃ substrate at a predetermined temperature by the heater electrode, and applies high-frequency power to the target electrode and also applies high-frequency bias power to the bias electrode, and wherein the high-frequency power and the high-frequency bias power are applied so that frequency interference between the high-frequency power and the high-frequency bias power does not occur.

Moreover, a manufacturing method of a semiconductor light-emitting element according to a second aspect of the present invention has a step of forming a buffer layer of the semiconductor light-emitting element by the method of forming an epitaxial film according to the above-described first aspect.

Moreover, a third aspect of the present, invention is a semiconductor light-emitting element in which at least a buffer layer, a group III nitride semiconductor intermediate layer, an n-type group III nitride semiconductor layer, a group III nitride semiconductor active layer, a p-type group III nitride semiconductor layer, and a translucent electrode are formed on the α-Al₂O₃ substrate, wherein at least one layer of the buffer layer, the group III nitride semiconductor intermediate layer, the n-type group III nitride semiconductor layer, the group III nitride semiconductor active layer, and the p-type group III nitride semiconductor layer is fabricated by the epitaxial film forming method according to the above-described first aspect. Moreover, a fourth aspect of the present invention is an illumination device characterized by including the semiconductor light-emitting element according to the above-described third aspect.

Moreover, the fourth aspect of the present invention is a sputtering apparatus comprising: a target electrode on which a target can be arranged; a substrate holder on which a substrate can be arranged toward the target electrode and provided with a heater electrode and a bias electrode; and frequency interference suppressing means for preventing frequency interference from occurring between the high-frequency power applied to the target electrode and the high-frequency bias power applied to the bias electrode when the step of forming an epitaxial film of the group III nitride semiconductor thin film according to the above-described first aspect is performed.

According to the present invention, an epitaxial film of a group III nitride semiconductor in which mosaic expansion of tilt or twist is small, mixture of +c polarity and −c polarity is reduced, and unification of the +c polarity is improved can be fabricated on the α-Al₂O₃ substrate by using the sputtering method. Moreover, by using the group III nitride semiconductor epitaxial film fabricated by this sputtering method, light emission characteristics of light-emitting elements such as LED and LD can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional schematic diagram of a high-frequency sputtering apparatus according to one embodiment of the present invention.

FIG. 2 is a first configuration example of a substrate holder according to one embodiment of the present invention.

FIG. 3 is a second configuration example of a substrate holder according to one embodiment of the present invention.

FIG. 4 is a third configuration example of a substrate holder according to one embodiment of the present invention.

FIG. 5 is a diagram illustrating a model in which a group III nitride semiconductor thin film with +c polarity is formed according to one embodiment of the present invention.

FIG. 6 is a sectional view illustrating an example of an LED structure fabricated by using an epitaxial film formed by an epitaxial film forming method according to one embodiment of the present invention.

FIG. 7A is a diagram for explaining frequency interference suppressing means according to one embodiment of the present invention.

FIG. 7B is a diagram for explaining the frequency interference suppressing means according to one embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be described below in detail. In the figures which will be described below, the same reference numerals are given to those having the same functions and duplicated explanation will be omitted.

Major features of the present invention is that, when a group III nitride semiconductor thin film is to be epitaxially grown on an α-Al₂O₃ substrate by using a sputtering method such as a high-frequency sputtering method, for example, the α-Al₂O₃ substrate is heated to an arbitrary temperature by a substrate holder provided with a heater electrode and a bias electrode and then, a film of the group III nitride semiconductor is formed while a high-frequency bias power is applied to the bias electrode. The present invention will be described by referring to the attached drawings. Members, arrangements and the like which will be described below are one example which embodies the invention and do not limit the present invention and they can be naturally modified in various ways in accordance with the gist of the present invention.

FIG. 1 is a schematic configuration diagram illustrating an example of a sputtering apparatus used in forming a group III nitride semiconductor thin film according to an embodiment of the present invention. In FIG. 1 illustrating a sputtering apparatus 1, reference numeral 101 denotes a vacuum vessel, reference numeral 102 denotes a target electrode, reference numeral 103 denotes a bias electrode, reference numeral 104 denotes a heater electrode, reference numeral 105 denotes a target shield, reference numeral 106 denotes a high-frequency power supply for sputtering, reference numeral 107 denotes a substrate, reference numeral 108 denotes a target, reference numeral. 109 denotes a gas introduction, mechanism, reference numeral 110 denotes an evacuation mechanism, reference numeral 111 denotes a substrate holder, reference numeral 112 denotes a reflector, reference numeral 113 denotes an insulating material, reference numeral 114 denotes a chamber shield, reference numeral 115 denotes a magnet unit, reference numeral 116 denotes a target-shield holding mechanism, and reference numeral 130 denotes a high-frequency power supply for bias, respectively. Reference numeral 111 in FIG. 1 is assumed to be arbitrary one of substrate holders 111 a, 111 b, and 111 c which will be described later. Moreover, as the substrate 107, an α-Al₂O₃ substrate (601) can be used.

The vacuum vessel 101 is formed of a metal member such as stainless and aluminum alloy and is electrically grounded. Moreover, the vacuum vessel 101 prevents or reduces temperature rise of a wall surface by a cooling mechanism, not shown. Moreover, the vacuum vessel 101 is connected to the gas introduction mechanism 109 through a mass flow controller, not shown, and is connected to the evacuation mechanism 110 through a variable conductance valve, not shown.

The target shield 105 is mounted on the vacuum vessel 101 through the target-shield holding mechanism 116. The target-shield holding mechanism 116 and the target shield 105 can be made of a metal member such as stainless and aluminum alloy and is electrically connected to the vacuum vessel 101.

The target electrode 102 is mounted on the vacuum vessel 101 through the insulating material 113. Moreover, the target 108 is mounted on the target electrode 102, and the target electrode 102 is connected to the high-frequency power supply 106 for sputtering through a matching box, not shown. The target 108 may be directly mounted on the target electrode 102 or may be mounted on the target electrode 102 through a bonding plate, not shown, made of a metal member such as copper (Cu). Moreover, the target 108 may be a metal target containing at least one of Al, Ga, or In or a nitride target containing at least, one of the above-described group III elements. On the target electrode 102, a cooling mechanism, not shown, for preventing temperature rise of the target 108 is provided. Moreover, the target electrode 102 incorporates the magnet unit 115, As power to be input to the target electrode 102 from the high-frequency power supply 106 for sputtering, industrially 13.56 MHz can be used easily, but it is also possible to use high frequency of another frequency, to superimpose a direct current on a high frequency or to use them in pulse.

The chamber shield 114 is mounted on the vacuum vessel 101 and prevents or reduces adhesion of a film to the vacuum vessel 101 during film formation. The substrate holder 111 has the heater electrode 104 and the bias electrode 103 therein. To the heater electrode 104, a power supply for heating, not shown, is connected, while to the bias electrode 103, a high-frequency power supply 130 for bias is connected through a matching box, not shown.

FIGS. 2 to 4 illustrate the configuration examples 111 a, 111 b, and 111 c of the substrate holder 111, respectively, and reference numeral M in each figure denotes a substrate placement surface. In FIG. 2 (or FIG. 3), reference numeral 201 denotes a base, reference numeral 202 denotes a base coat, reference numeral 103 a (or reference numeral 103 b) denotes a bias electrode, reference numeral 104 denotes the heater electrode, and reference numeral 205 denotes an over coat. The base 201 is made of graphite, the base coat 202 and the over coat 205 are made of pyrolytic boron nitride (PBN), and the bias electrode 103 a (or 103 b) and the heater electrode 104 are made of pyrolvtic graphite (PG), and the base coat 202 and the over coat 205 made of PBN are dielectric materials with high resistance.

In FIG. 2 (or FIG. 3), a power supply for heating, not shown, is connected to the heater electrode 104. Joule heat is generated by having an AC or DC current flow through the heater electrode 104, and the α-Al₂O₃ substrate placed on the substrate holder 111 a (or 111 b) can be heated by infrared rays from the substrate holder heated by the Joule heat.

Moreover, in FIG. 2 (or FIG. 3), the high-frequency power supply 130 for bias is connected to the bias electrode 103 a (or 103 b) through a matching box, not shown. By applying high-frequency bias power to the bias electrode 103 a (or 103 b) during film formation, a negative DC bias voltage can be generated, on the surface of the α-Al₂O₃ substrate placed on the substrate holder 111 a (or 111 b).

It is also possible to further connect a power supply for an electrostatic chuck (ESC), not shown, to the bias electrode 103 a (or 103 b) in FIG. 2 (or FIG. 3) through a low-pass filter, not shown. In such a case, the bias electrode 103 a is configured as electrically separated electrodes as indicated by reference numeral A and reference numeral B (one is a first electrode, while the other is a second electrode), and a bipolar ESC may be realized by applying positive and negative DC voltages to the electrodes, respectively. In this way, since the α-Al₂O₃ substrate can be electrostatically adsorbed to the substrate holder, the α-Al₂O₃ substrate placed on the substrate holder 111 a can be efficiently heated. With regard to the subscrate holder 111 b, too, the bias electrode 103 b is configured as electrically separated electrodes as indicated by reference numeral C and reference numeral D (one is a first electrode, while the other is a second electrode) and a bipolar ESC can be realized by applying positive and negative DC voltages to the electrodes, respectively.

FIG. 4 is another configuration example 111 c of the substrate holder 111. Reference numeral 401 denotes a base, reference numeral 402 denotes a base coat, reference numeral 403 denotes a common electrode, reference numeral 404 denotes a backside coat, and reference numeral 405 denotes an over coat. The base 401 is made of graphite, the base coat 402 and the over coat 405 are made of PBN, the common electrode 403 and the backside coat 404 are made of PG, and the base coat 402 and the over coat 405 made of PBN are dielectric materials with high resistance.

In FIG. 4, to the common electrode 403, the high-frequency power supply 130 for bias is connected through a matching box, and moreover, a power supply for heating, not shown, is connected through a low-pass filter, not shown.

In FIG. 4, the common electrode 403 has a function integrating the heater electrode 104 and the bias electrode 103 a in FIG. 2. By having an AC or DC current flow from the power supply for heating to the common electrode 403, the substrate holder 111 c produces heat, and the α-Al₂O₃ substrate placed on the substrate holder 111 c is heated by the infrared rays thereof. Moreover, by further applying high-frequency bias power from the high-frequency power supply for bias in a state where the current for heating is made to flow through the common electrode 403, a negative DC bias voltage can be generated on the surface of the α-Al₂O₃ substrate, while the α-Al₂O₃ substrate placed on the substrate holder 111 c is heated. In this way, the effect of the present invention can be also obtained by using the common electrode integrating the heater electrode and the bias electrode into one.

In the substrate holder 111 a having a structure illustrated in FIG. 2, Joule heat generated from the heater electrode 104 is transmitted to the substrate placement surface M side through the base coat 202, the base 201, the over coat 205, and the bias electrode 103 a. At this time, particularly since the base 201 plays a role as a soaking plate, high soaking performance can be easily obtained in the substrate plane.

Moreover, in the substrate holder 111 b having a structure illustrated in FIG. 3, the bias electrode 103 b is formed of a substantially disk-shaped electrode (corresponding to reference numeral C) at the center part and a substantially ring-shaped electrode (corresponding to reference numeral D) on an outer periphery part. Thus, the bias electrode 103 b (particularly the reference numeral C part) further plays a role of a soaking plate and can further improve the soaking performance in the plane more than the substrate holder 111 a having the structure illustrated in FIG. 2. Particularly, if the α-Al₂O₃ substrate is adsorbed by the ESC, temperature distribution depending on a pattern shape of the bias electrode 103 a might be generated in the substrate holder 111 a having the structure illustrated in FIG. 2, but with the structure as in FIG. 3, such problem can be markedly improved.

By using the ESC, a temperature rising speed after the α-Al₂O₃ substrate is placed on the substrate holders 111 a and 111 b can be increased and thus, use of the ESC is a preferable mode in obtaining high productivity.

In the substrate holder 111 c having a structure illustrated in FIG. 4, Joule heat generated from the common electrode 403 is transmitted to the substrate placement surface M side not through the base 401 or the base coat 402. Thus, as compared with the substrate holders illustrated in FIGS. 2 and 3, it is difficult to obtain high soaking performance. On the other hand, since the α-Al₂O₃ substrate is heated not though the base 401 or the base coat 402, temperature gradient of the substrate placement surface M and the common electrode 403 becomes small, and the substrate can be heated with high power use efficiency without using the ESC.

The material constituting the substrate holder illustrated in FIGS. 2 to 4 is preferably used since efficiency to heat the α-Al₂O₃ substrate is higher than a prior-art infrared lamp, but this is not limiting as long as the α-Al₂O₃ substrate can be heated to a predetermined temperature.

Moreover, the substrate holder is not limited to the structures of the above-described substrate holders 111 a, 111 b, and 111 c. The structures of the above-described substrate holders 111 a, 111 b, and 111 c are preferable modes since they can improve soaking performance and power use efficiency, and thus the structure can be selected as appropriate in accordance with the purpose. However, in the present invention, it is important that a negative DC bias voltage is generated on the surface of the α-Al₂O₃ substrate by applying high-frequency bias power to the bias electrode at a predetermined temperature, and as a result, an epitaxial film of the group III nitride semiconductor can be formed with +c polarity. Therefore, it is needless to say that any structure can be applied to this embodiment, as long as the structure accords with the gist of the present invention.

FIG. 5 is a model, diagram illustrating a mechanism in which the group III nitride semiconductor thin film is formed with +c polarity by applying high-frequency bias power to the bias electrode. In FIG. 5, reference numeral 111 denotes an arbitrary substrate holder in the substrate holders 111 a, 111 b, and 111 c, and reference numeral 107 is an α-Al₂O₃ substrate, and reference numeral 503 is a nitride molecule.

FIG. 6 is an example of a sectional structure of a light-emitting diode (LED) as a semiconductor light-emitting element fabricated by using the manufacturing method of the group III nitride semiconductor thin film according to the embodiment of the present invention. In FIG. 6, reference numeral 601 denotes an α-Al₂O₃ substrate, reference numeral 602 denotes a buffer layer, reference numeral 603 denotes a group III nitride semiconductor intermediate layer, reference numeral 604 denotes an n-type group III nitride semiconductor layer, reference numeral 605 denotes a group III nitride semiconductor active layer, reference numeral 606 denotes a p-type group III nitride semiconductor layer, reference numeral 607 denotes a translucent electrode, reference numeral 608 denotes an n-type electrode, reference numeral 609 denotes a p-type bonding pad electrode, and reference numeral 610 denotes a protective film.

As a material constituting the buffer layer 602, AlN, AlGaN, and GaN are preferably used. As a material constituting the group III nitride semiconductor intermediate layer 603, the n-type group III nitride semiconductor layer 604, the group III nitride semiconductor active layer 605, and the p-type group III nitride semiconductor layer 606, AlGaN, GaN and InGaN are preferably used. It is preferable that silicon (Si) or germanium (Ge) is added m an extremely small amount in the above-described material for the n-type group III nitride semiconductor layer 604, and magnesium (Mg) or zinc (Zn) is added in an extremely small amount in the above-described material for the p-type group III nitride semiconductor layer 606, respectively, for executing control of conductivity. Moreover, the group III nitride semiconductor active layer 605 preferably forms a multiple quantum well (MQW) structure of the above-described materials. Furthermore, an illumination device can be configured by using the above-described light-emitting diode (LED).

FIGS. 7A and 7B are diagrams for explaining frequency interference suppressing means according to the embodiment of the present invention.

FIG. 7A is an example of means for suppressing frequency interference (frequency interference suppressing means) which will be described later by using high-frequency power supplies at different frequencies as the high-frequency power supply 106 for sputtering and the high-frequency power supply 130 for bias. Reference numerals 701 and 702 illustrate matching boxes. The high-frequency power from the high-frequency power supply 106 for sputtering is supplied to the target electrode 102 with a reflective wave being reduced by going through the matching box 701, while the high-frequency power from the high-frequency power supply 130 for bias is supplied to the bias electrode 103 with a reflective wave being reduced by going through the matching box 702. The high-frequency power supply 106 for sputtering and the high-frequency power supply 130 for bias are set so as to have different frequencies. For example, if the frequency of the high-frequency power supply 106 for sputtering is set to 13.56 MHz, by using a frequency of 13.54 MHz or 13.58 MHz as the high-frequency power supply 130 for bias, the frequency interference which will be described later can be suppressed.

FIG. 7B illustrates an example of means for suppressing frequency interference which will be described later (frequency interference suppressing means) by adjusting a phase of the high-frequency power from the high-frequency power supply 106 for sputtering and from the high-frequency power supply 130 for bias. In FIG. 7B, reference numeral 703 denotes a phase control unit, reference numeral 704 denotes a high-frequency oscillator, and reference numeral 705 and reference numeral 706 denote phase adjustment circuits. The high-frequency power from the high-frequency power supply 106 for sputtering is supplied to the target electrode 102 with a reflective wave being reduced by going through the matching box 701, while the high-frequency power from the high-frequency power supply 130 for bias is supplied to the bias electrode 103 with a reflective wave being reduced by going through the matching box 702.

The phase control unit 703 has the high-frequency oscillator 704 and the phase adjustment circuits 705 and 706, and can adjusts a phase of a high-frequency signal from the high-frequency oscillator 704 by the phase adjustment circuits 705 and 706 to output to an external circuit. Moreover, an output portion of the phase control unit 703 is connected to an external input terminal portion of the high-frequency power supply 106 for sputtering and the high-frequency power supply 130 for bias. By means of a high-frequency signal which is output by the phase control unit 703 and the phase of which is adjusted (that is, a high-frequency signal oscillated by the high-frequency oscillator 704, and moreover a high-frequency signal whose phase is adjusted by the phase adjustment circuits 705 and 706), a phase of the high-frequency power output from the high-frequency power supply 106 for sputtering and the high-frequency power supply 130 for bias is controlled. For example, by adjusting the phase control unit 703 and by setting a phase difference of the high-frequency power output from the high-frequency power supply 106 for sputtering and the high-frequency power supply 130 for bias to a phase difference of 180° or the like, frequency interference which will be described later can be suppressed.

In order to prevent frequency interference which will be described later from occurring as described above, setting the high-frequency power supplied to the target electrode 102 and the high-frequency power supplied to the bias electrode 103 so as to have different frequencies or keeping the high-frequency power supplied to the target electrode 102 and the high-frequency power supplied to the bias electrode 103 at a predetermined phase difference are effective means. In order to obtain the effect of the present invention with high reproducibility, provision of these frequency interference suppressing means is extremely effective.

An epitaxial film forming method of a group III nitride semiconductor thin film by using the sputtering apparatus according to the present invention will be described below by referring to the attached drawings. In this embodiment, an epitaxial film is formed on an α-Al₂O₃ substrate by a method having the following first to fourth steps. In the following description, the substrate holder 111 is assumed to refer to an arbitrary one of the substrate holders 111 a, 111 b, and 111 c, and the bias electrode 103 is assumed to refer to the bias electrodes 103 a, 103 b, and 403 (403 is a common electrode) provided in the arbitrary substrate holders 111 (111 a, 111 b, and 111 c).

First, as the first step, the substrate 107 is introduced into the vacuum vessel 101 maintained at a predetermined pressure by the evacuation mechanism 110. At this time, the substrate (α-Al₂O₃ substrate) 107 is conveyed to the. upper part of the substrate holder 111 by a conveyer robot, not shown, and is held at an upper part of a lift pin, not shown, protruding from the substrate holder 111. After that, the lift pin holding the substrate 107 is lowered, and the substrate 107 is placed on the substrate holder 111.

Subsequently, as the second step, a voltage to be applied to the heater electrode 104 incorporated in the substrate holder 111 is controlled so as to maintain the substrate 107 at a predetermined temperature. At this time, control is executed such that the temperature of the substrate holder 111 is monitored by using a thermocouple (not shown) incorporated in the substrate holder 111 or the temperature of the substrate holder 111 is monitored by using a pyrometer, not shown, installed in the vacuum vessel 101 so that the temperatures become predetermined temperatures.

Subsequently, as the third step, either of an N₂ gas or a mixed gas of an N₂ gas and a rare gas is introduced into the vacuum vessel 101 from the gas introduction mechanism 109, and the pressure of the vacuum vessel 101 is set to a predetermined pressure by a mass flow controller (not shown) and a variable conductance valve (not shown).

Lastly, as the fourth step, by applying a high-frequency bias power to the bias electrode 103 incorporated in the substrate holder 111 and by applying a high-frequency power to the target 108 by the high-frequency power supply 106 for sputtering, plasma is generated on a front surface of the target 108. As a result, an ion in the plasma throws out elements constituting the target 108, and a group III nitride semiconductor thin film is formed on the substrate 107 by the thrown out elements.

The predetermined pressure in the first step is preferably less than 5×10⁻⁴ Pa and if the pressure is not smaller than that, many impurities such as oxygen and the like are taken into the group III nitride semiconductor thin film, and it is difficult to obtain a favorable epitaxial film. Moreover, the temperature of the substrate holder 111 in the first step is not particularly limited, but from the viewpoint of productivity, a temperature for obtaining a substrate temperature during film forming is preferably set.

The predetermined temperature in the second step is preferably set to a film forming temperature in the fourth step from the viewpoint of productivity, and the predetermined pressure in the third step is preferably set to a film forming pressure in the fourth step from the viewpoint of productivity. Operation timings of the second step and the third step may be exchanged or they may be performed at the same time. Moreover, the temperature set in the second step and the pressure set in the third step are preferably maintained at least until the fourth step is started from the viewpoint of productivity.

In the fourth step, the high-frequency bias power to be applied to the bias electrode 103 needs to be set to predetermined power with which a group III nitride semiconductor film (a group III nitride semiconductor thin film with +c polarity) with high unification degree of +c polarity can be obtained. If power is too small, a group III nitride semiconductor thin film in which polarities are mixed is formed, while if power is too large, the group III nitride semiconductor thin film is damaged by collision with high-energy particles, and a good quality group III nitride semiconductor thin film cannot be obtained.

In this description, a group III nitride semiconductor thin film in which −c polarity is not present or reduced, that is, a group III nitride semiconductor thin film in which mixture of the +c polarity and the −c polarity is reduced, and a unification degree of the +c polarity is high is called “a group III nitride semiconductor thin film with +c polarity”.

Moreover, the substrate temperature when the fourth step is performed is preferably set within a range of 100 to 1200° C., or more preferably within a range of 400 to 1000° C. If it is less than 100° C., a film in which an amorphous structure is mixed is formed easily, while if it is higher than 1200° C., a film itself is not formed or even if a film is formed, an epitaxial film with many defects is obtained easily due to heat stress. Moreover, a film forming pressure is preferably set within a range of 0.1 to 100 mTorr (1.33×10⁻² to 1.33×10¹ Pa), or more preferably set within a range of 0.1 to 10 mTorr (1.33×10⁻¹ to 1.33 Pa).

If it is less than 0.1 mTorr (1.33×10⁻² Pa), high energy particles can easily enter the substrate surface, and thus it is difficult to obtain a good quality group III nitride semiconductor thin film, while if the pressure is higher than 100 mTorr (1.33×10¹ Pa), a film-forming speed becomes extremely low, which is not preferable. When the fourth step is to be started, it is possible to temporarily raise the pressure of the vacuum vessel 101 to the film-forming pressure or more so as to promote generation of plasma on the target side. In this case, the film-forming pressure may be raised by temporarily introducing a large gas flow rate of at least one type of process gases or by temporarily decreasing an opening degree of the variable conductance valve (not shown).

In the fourth step, the timing to apply high-frequency bias power to the bias electrode 103 and the timing to apply high-frequency power to the target electrode 102 may be the same or may be set such that one is applied first and then, the other is applied. However, if the high-frequency power is to be applied to the target electrode 102, first, the high-frequency bias power needs to be applied to the bias electrode 103 before a film formation surface of the α-Al₂O₃ substrate 107 is covered by a crystal layer made of a group III nitride semiconductor.

The crystal layer of the group III nitride semiconductor formed in a state where the high-frequency bias power is not applied to the bias electrode 103 is easily brought into a state in which polarities are mixed or a state of −c polarity. If a state in which the −c polarity is mixed occurs, it is difficult to obtain the group III nitride semiconductor thin film with +c polarity even if the high-frequency bias power is applied to the bias electrode 103 after that. Therefore, if the high-frequency power is to be applied to the target electrode 102, first, it is preferable that the high-frequency power is applied to the target electrode 102 and immediately after plasma is generated on the front surface of the target (that is, after sputtering is started),the high-frequency bias power is applied to the bias electrode 103 and before a crystal layer made of the group III nitride

semiconductor is formed on the α-Al₂O₃ substrate 107, the high-frequency bias power is applied to the bias electrode 103.

If the high-frequency bias power is to be applied to the bias electrode 103, first, plasma is generated on the surface side of the α-Al₂O₃ substrate 107 during a period until the high-frequency power is applied to the target electrode 102, and surface nitridization of the α-Al₂O₃ substrate 107 caused by an active species containing an N atom in the plasma needs to be avoided. That is because, if the surface of the α-Al₂O₃ substrate 107 is nitrided, AlN with −c polarity or with mixed polarity is easily formed on the substrate surface and even if the high-frequency power is applied to the target, electrode 102 after that so as to form the group III nitride semiconductor thin film, it becomes difficult to obtain a group III nitride semiconductor thin film with +c polarity. Therefore, if the high-frequency bias power is to be applied to the bias electrode 103, first, it is preferable that the high-frequency power is applied to the target electrode 102 immediately after the high-frequency bias power is applied to the bias electrode 103 and then, sputtering is started.

Moreover, it is needless to say that before the first step, such a step may be provided in which the substrate 107 is conveyed to a pre-treatment chamber (not shown) for thermal treatment or plasma processing of the substrate 107 at a temperature of the film-forming temperature or more. However, if the plasma processing is to be performed, it is important to select a condition that an AlN layer with mixed polarity or an AlN layer with −c polarity is not formed on the surface of the α-Al₂O₃ substrate.

A mechanism of forming a group III nitride semiconductor thin film with +c polarity by the above-described first to fourth steps will be described below by using FIG. 5. As the first and second steps, the α-Al₂O₃ substrate 107 is placed on the substrate holder 111 so that the substrate is at a predetermined temperature, and as the third step, either of an N₂ gas or a mixed gas of an N₂ gas and a rare gas is introduced into the vacuum vessel. Subsequently, as the fourth step, high-frequency bias power is applied to the bias electrode, and also plasma is generated on the target side so as to form a group III nitride semiconductor thin film.

If a metal target is used as a target in the fourth step, it is considered that the target surface is nitrided by an active species containing an N atom, and by sputtering the surface with a positive ion, a nitride molecule 503 illustrated in FIG. 5 is emitted from the target surface to reach the surface of the α-Al₂O₃ substrate 107. Moreover, even if a nitride target is used, it is considered that by sputtering the surface with a positive ion, a nitride molecule 503 illustrated in FIG. 5 is emitted from the target surface to reach the surface of the α-Al₂O₃ substrate 107. A nitride molecule 503 of a 2-atom molecule is illustrated in FIG. 5 for simplification, but the molecule is not limited to a 2-atom molecule as long as it is a nitride molecule.

In FIG. 5, high-frequency bias power is applied to the bias electrode 103, and in a space facing the surface side of the α-Al₂O₃ substrate 107, a plasma region indicated by reference numeral G and a sheath region indicated by reference numeral S are formed. The sheath region S is formed between the plasma region G and the α-Al₂O₃ substrate 107.

In the plasma region G, densities of a positive charge (positive ion) and a negative charge (electron) are substantially equal and substantially in an electrically neutral state. Moreover, the plasma region G is usually in a substantially constant potential state (called plasma potential) which is positive with respect to grounding potential. On the other hand, since a positive ion and an electron are different in ease of follow-up with respect to a change in a high-frequency electric field generated by application of the high-frequency bias power, excessive electrons are supplied to the surface of the α-Al₂O₃ substrate 107, and a negative DC bias voltage is generated. In the sheath region S, an electric field indicated by reference numeral E in a direction toward the surface of the α-Al₂O₃ substrate 107 is generated by a potential difference between the negative potential on the surface of the α-Al₂O₃ substrate 107 generated as above and the plasma potential of the plasma region G. The size of this electric field E can be adjusted by magnitude of the high-frequency bias power.

As a mode of power to be applied to the bias electrode 103, high-frequency power is preferable. In the case of DC power, since the α-Al₂O₃ substrate 107 is an insulating material, it becomes difficult to effectively generate a negative DC bias voltage on the surface of the α-Al₂O₃ substrate 107, which is not preferable.

The nitride molecule 503 has a group III element 503 a and an N atom 503 b, and the group III element 503 a has positive charge biased and the N atom 503 b has negative charge biased. That is, the nitride molecule 503 has polarization indicated by reference numeral P, This nitride molecule 503 is considered to be oriented in random directions in the plasma region G, but when it reaches the sheath region S, the electric field E acts on the polarization P of the nitride molecule 503, and it is considered that the group III element 503 a is oriented in the direction of the α-Al₂O₃ substrate and the N atom 503 b is oriented in the direction of the plasma region G, that is, the polarization P is oriented to the direction of the α-Al₂O₃ substrate.

In the group III nitride semiconductor with +c polarity, the polarization P of the nitride molecule 503 is oriented to the direction of the α-Al₂O₃ substrate. That is, it is considered that the polarization P of the nitride molecule 503 is oriented to the direction of the α-Al₂O₃ substrate by the electric field E of the sheath region S generated by application of the high-frequency bias power, and by means of adsorption to the α-Al₂O₃ substrate surface while the orientation is maintained, a group III nitride semiconductor thin film with +c polarity Is obtained.

Even if the high-frequency bias power is applied to the bias electrode 103, if the high-frequency bias power is small, a group III nitride semiconductor with +c polarity might not be obtained. The reason for that is considered to be that the electric field E of the sheath region S did not sufficiently act on the polarization P of the nitride molecule 503 and could not control orientation.

Moreover, if the high-frequency bias power is too large, a high-quality group III nitride semiconductor might not be obtained. The reason for that is considered to be that the positive ion in the plasma is accelerated by the electric field E in the sheath region S and collides against the surface of the α-Al₂O₃ substrate with large energy, and thus many defects are formed inside the group III nitride semiconductor thin film.

As described above, in order to obtain a group III nitride semiconductor thin film with +c polarity, it is necessary to adjust the magnitude of the high-frequency bias power to be applied to the bias electrode 103 to an appropriate value. An optimal range of this high-frequency bias power is different largely depending on the internal structure of the sputtering apparatus, and thus an optimal condition needs to be acquired for each apparatus.

Moreover, a frequency used as the high-frequency bias power is not particularly limited, but if the frequency of the high-frequency bias power and the frequency of the high-frequency power applied to the target match each other, a low-frequency beat phenomenon caused by interference of high-frequency power may easily occur, and a film-forming condition might be also affected (hereinafter this low-frequency beat phenomenon is called frequency interference). If frequency interference occurs in this embodiment, plasma becomes unstable and the DC bias voltage generated on the surface of the α-Al₂O₃ substrate no longer becomes stable, and thus high frequency power at a different frequency is preferably used. By using FIG. 1A as an example, assuming that a frequency of high-frequency power to be applied to the target electrode 102 (a frequency of the high-frequency power supply 106 for sputtering) is 13.56 MHz, by using 13.54 MHz or 13.58 MHz for the frequency of the high-frequency bias power (frequency of the high-frequency power supply 130 for bias) to be applied to the bias electrode 103, the above-described frequency interference can be prevented or reduced.

Moreover, by shifting the high-frequency bias power to be applied to the bias electrode and the high-frequency power to be applied to the target by a predetermined phase difference, the above-described frequency interference can be suppressed. By using FIG. 7B as an example, if a phase difference between the high-frequency bias power to be applied to the bias electrode 103 and the high-frequency power to be applied to the target electrode 102 is adjusted to be 180° by the phase control, unit 703, that is, if adjustment is made so that a positive peak top voltage of the high-frequency power is applied to the target electrode 102 and at the same time, a negative peak top voltage of the high-frequency bias power is applied to the bias electrode 103, frequency interference can be prevented or reduced most effectively. Moreover, the phase difference may be finely adjusted so that a reflective wave to each of the high-frequency power supplies (high-frequency power-supply for sputtering and high-frequency power supply for bias) is further reduced. That is, it is assumed that the phase difference of 180° includes a range of fine adjustment.

Moreover, even in the case of other phase difference, it can be used without problem unless frequency interference is caused. If frequency interference as above occurs, the plasma becomes unstable and a reflective wave to each of the high-frequency power supplies (high-frequency power supply for sputtering and high-frequency power supply for bias) may easily increase, and thus it is preferable that adjustment is made to such a phase difference that minimizes the frequency interference (preferably to zero).

Even if the high-frequency bias power is not applied to the bias electrode 103, the electric field E is generated in the sheath region S, but the electric field E generated at this time is generally smaller than that generated when the high-frequency bias power is applied. Therefore, the reason why a nitride semiconductor thin film with +c polarity cannot, be obtained if the high-frequency bias power is not applied to the bias electrode 103 is considered to be that the electric field E of the sheath region S does not sufficiently act on the polarization P of the nitride molecule 503 and cannot control orientation.

When the metal target 108 is to be sputtered by plasma using a mixed gas of an N₂ gas and a rare gas, a ratio in the mixed gas of the N₂ gas and the rare gas should be controlled so that a metal component (non-nitride component) is not taken in the group III nitride semiconductor thin film in a large amount. If a large amount of the metal component is taken in, the ratio of a metal atom or the group III element emitted from the target in a metal cluster state tends to become larger than the nitride molecule 503, and thus it is likely that the effect of the present invention cannot be fully obtained even if the high-frequency bias power is applied to the bias electrode 103.

As the epitaxial film of the group III nitride semiconductor thin film formed by the method in this embodiment, the buffer layer 602, the group III nitride semiconductor intermediate layer 603, the n-type group III nitride semiconductor layer 604, the group III nitride semiconductor active layer 605, and the p-type group III nitride semiconductor layer 606 illustrated in FIG. 6 can be cited. All of the above-described layers may be fabricated by using the sputtering apparatus (epitaxial film forming method) according to the present invention or only limited layers may be fabricated by using the sputtering apparatus (epitaxial film forming method) according to the present invention.

As a first example, for example, there is a method of fabricating an epitaxial wafer by fabricating the buffer layer 602 of an LED element in FIG. 6 by using the sputtering apparatus (epitaxial film forming method) according to the present invention and then, by sequentially laminating the group III nitride semiconductor intermediate layer 603, the n-type group III nitride semiconductor layer 604, the group III nitride semiconductor active layer 605, and the p-type group III nitride semiconductor layer 606 by using the MOCVD method.

As a second example, there is a method of fabricating an epitaxial wafer by fabricating the buffer layer 602 and the group III nitride semiconductor intermediate layer 603 by using the sputtering apparatus (epitaxial film forming method) according to the present invention and then, by sequentially laminating the n-type group III nitride semiconductor layer 604, the group III nitride semiconductor active layer 605, and the p-type group III nitride semiconductor layer 606 by using the MOCVD method.

As a third example, there is a method of fabricating an epitaxial wafer by fabricating the buffer layer 602, the group III nitride semiconductor intermediate layer 603, and the n-type group III nitride semiconductor layer 604 by using the sputtering apparatus (epitaxial film forming method) according to the present invention and then, by sequentially laminating the group III nitride semiconductor active layer 605 and the p-type group III nitride semiconductor layer 606 by using the MOCVD method.

As a fourth example, there is a method of fabricating an epitaxial wafer by fabricating the buffer layer 602, the group III nitride semiconductor intermediate layer 603, the n-type group III nitride semiconductor layer 604, and the group III nitride semiconductor active layer 605 by using the sputtering apparatus (epitaxial film forming method) according to the present invention and then, by fabricating the p-type group III nitride semiconductor layer 606 by using the MOCVD method.

As a fifth example, there is a method of fabricating an epitaxial wafer by fabricating the buffer layer 602, the group III nitride semiconductor intermediate layer 603, the n-type group III nitride semiconductor layer 604, the group III nitride semiconductor active layer 605, and the p-type group III nitride semiconductor layer 606 by using the sputtering apparatus (epitaxial film forming method) according to the present invention.

On the epitaxial wafer obtained as above, by forming the translucent electrode 607, the p-type bonding pad electrode 609, the n-type electrode 608, and the protective film 610 as illustrated in FIG. 6 by using the lithography technology and RIE (reactive ion etching) technology, an LED structure can be obtained. The material for the translucent electrode 607, the p-type bonding pad electrode 609, the n-type electrode 608, and the protective film 610 is not particularly limited but materials well-known in this technical field can be used without limitation.

EXAMPLE First Example

As a first example of the present invention, an example in which an AlN film as the buffer layer 602 (See FIG. 6) was formed on an α-Al₂O₃ (0001) substrate by using the film-forming method of a group III nitride semiconductor thin film according to an embodiment of the present invention will be described. More specifically, an example in which, in a state where the high-frequency bias power was applied to the bias electrode 103, the AlN film was formed on the α-Al₂O₃ (0001) substrate by using the sputtering method will be described. In the first example, the AlN film was formed by using the sputtering apparatus similar to that in FIG. 1. Moreover, the frequencies of the high-frequency power to be applied to the target electrode 102 and the high-frequency power to be applied to the bias electrode 103 are set to 13.56 MHz and 13.54 MHz, respectively.

In the first example, first, the α-Al₂O₃ (0001) substrate was conveyed to the vacuum vessel 101 maintained at 1×10⁻⁴ Pa or less and placed on the substrate holder 111 by the first step, and the substrate was maintained at 550° C. which was a film-forming temperature by the second step. At this time, a current made to flow to the heater electrode 104 was controlled so that a monitor value of the thermocouple incorporated in the substrate holder 111 was at 750° C.

Subsequently, by the third step, a mixed gas of an N₂ gas and an Ar gas was introduced so that N₂/(N₂+Ar): 25% was realized, and the pressure of the vacuum vessel 101 was set to 3.75 mTorr (0.5 Pa). In this state, the high-frequency bias power at 10 W was applied to the bias electrode 103 by the fourth step, and also the high-frequency power at 2000 W was applied from the high-frequency power supply 106 for sputtering to the target 108 made of metal Al, and an AlN film having a film thickness of 50 nm was formed on the substrate by the sputtering method. It was confirmed by X-ray photoelectron spectroscopy (XPS) at this time that the obtained AlN film contained almost no metal Al component.

The film-forming temperature in the first example was set from a relationship between the temperature of the α-Al₂O₃ (0001) substrate and the monitor value of the thermocouple incorporated in the heater at that time, that is, the temperature of the heater by conducting substrate temperature measurement in advance of the α-Al₂O₃ (0001) substrate in which the thermocouple was embedded.

In the first example, the fabricated AlN film was evaluated by X-ray diffraction (XRD) measurement in a 2θ/ω scan mode at a symmetric reflection position, XRC measurement in a ω scan mode for the symmetric plane, the XRC measurement in a φ scan mode in In-plane arrangement, and coaxial impact collision ion scattering spectroscopy (CAICISS) measurement. Here, the XRD measurement in the 2θ/ω scan mode at: a symmetric reflection position was used for checking crystal orientation, and the XRC measurement in the scan mode for the symmetric plane and the XRC measurement in the φ scan mode in In-plane arrangement were used for evaluation of mosaic expansion of tilt and twist, respectively. Moreover, the CAICISS measurement was used as means for determining polarity.

First, the XRD measurement in the 2θ/ω scan mode at a symmetric reflection position was conducted for the AlN film fabricated in the first example using a measurement range of 20=20 to 60°, only diffraction peaks of the AlN (0002) plane and the α-Al₂O₃ (0006) plane were observed, and a diffraction peak indicating other lattice planes of AlN was not observed. From this fact, it was known that the obtained AlN film had c-axis orientation.

Subsequently, the XRC measurement in the to scan mode for the symmetric plane (AlN (0002) plane in the first example) was conducted for the AlN film fabricated in the first example. The FWHM of the obtained XRC profile was 450 arcsec or less if a detector was in an open detector state and 100 arcsec or less if an analyzer crystal was inserted into the detector. Thus, it was confirmed that mosaic expansion of tilt in the fabricated AlN film was small. Moreover, depending on the fabrication conditions, those with the FWHM at 20 arcsec or less in the XRC measurement with the analyzer crystal inserted into the detector were also obtained.

The XRC measurement should be conducted by the detector in the open detector state, but in the case of a sample with a small film thickness as in the first example, the FWHM of the XRC profile expands due to film thickness effect or lattice relaxation, and accurate evaluation of mosaic expansion becomes difficult. Thus, in recent years, the case with the analyzer crystal inserted into the detector is also handled as the XRC measurement in a wide sense as above. Unless otherwise noted, it is assumed below that the open detector state is used in the XRC measurement.

Subsequently, the XRC measurement in the φ scan mode in In-plane arrangement was made for the AlN film fabricated in the first example. The AlN {10-10} plane was used for the measurement. Six diffraction peaks appeared at 60° intervals in the obtained XRC profile, and it was confirmed that the AlN film had six-time symmetry, that is, the AlN film epitaxially grew. Moreover, the FWHM acquired from the diffraction peak with the maximum magnitude was 2.0° or less, and it was known that mosaic expansion of twist of the fabricated AlN film was relatively small. When the α-Al₂O₃ (0001) substrate and the in-plane crystal orientation of the AlN film were compared, it was confirmed that an a-axis of the AlN film made in-plane rotation at 30° with respect to the a-axis of the α-Al₂O₃ (0001) substrate. This indicates that the AlN film was formed with a general epitaxial relationship when the AlN

film was epitaxially grown on the α-Al₂O₃ (0001) substrate.

Subsequently, for the AlN film fabricated in the first example, the CAICISS measurement was conducted. In this measurement, an Al signal was detected with a changed incident angle from an AlN [11-20] orientation, and it was known that a peak in the vicinity of the incident angle of 70° was obtained as a single shape. This indicates that the obtained AlN film has +c polarity.

As described above, it can be confirmed that the AlN film fabricated in the first example had +c polarity and is c-axis orientation epitaxial film with small mosaic expansion of tilt. That is, according to the present invention, it was made clear that a group III nitride semiconductor thin film with +c polarity could be obtained while mosaic expansion of tilt and twist was reduced. When the experiment similar to the first example was repeated several times, it was confirmed that reproducibility was favorable.

Second Example

Subsequently, as a second example of the present invention, an example in which the AlN film fabricated by using the film-forming method of the group III nitride semiconductor thin film according to the present invention was used as a buffer layer and an undoped GaN film as the group III nitride semiconductor intermediate layer 603 in FIG. 6 was formed on the buffer layer by using the MOCVD method will be described.

The AlN film was formed on the α-Al₂O₃ (0001) substrate by using the sputtering method with the same apparatus and conditions as those in the first example and then, a wafer (substrate) was introduced into the MOCVD device so as to form an undoped GaN film having a film thickness of 5 μm.

The surface of the obtained undoped GaN film was a mirror surface, and it was indicated in the XRD measurement in the 2θ/ω scan mode at a symmetric reflection position that the undoped GaN film had a c-axis orientation. Subsequently, the XRC measurement in the ω scan mode using the GaN (0002) plane as the symmetric plane and the XRC measurement in the φ scan mode in In-plane arrangement for the GaN {10-10} plane were conducted, and it was confirmed that the FWHMs were 250 arcsec or less and 500 arcsec or less, respectively. From this fact, it was known that the obtained undoped GaN film was obtained as a high-quality crystal with small mosaic expansion of tilt and twist. Moreover, from the CAICISS measurement, it was confirmed that the polarity of the obtained undoped GaN film was −c polarity. This can be considered to be the result that, as described in the first example, the polarity of the AlN film used as the buffer layer can be controlled to the +c polarity and thus, the undoped GaN film formed on the buffer layer also takes over the polarity.

As described above, by using the AlN film fabricated by using the film-forming method of the group III nitride semiconductor thin film according to the present invention and controlled to the +c polarity as the buffer layer, the undoped GaN film grown by using the MOCVD method on the buffer layer can be obtained as a high-quality epitaxial film controlled to +c polarity with small mosaic expansion. That is, a group III nitride semiconductor thin film with +c polarity can be epitaxially grown on the α-Al₂O₃ substrate.

In the second example, the undoped GaN film was formed by the MOCVD method, but it was confirmed that the similar result could be obtained by using the sputtering method, too. Moreover, when the experiment similar to the second example was repeated several times, it was confirmed that reproducibility was favorable.

Third Example

As a third example of the present invention, an example will be described in which the AlN film fabricated by using the film-forming method of the group III nitride semiconductor thin film according to the present invention was used as a buffer layer, and on the buffer layer, by using the MOCVD method, the group III nitride semiconductor intermediate layer made of undoped GaN, the n-type group III nitride semiconductor layer made of Si doped GaN, the group III nitride semiconductor active layer having an MQW structure of InGaN and GaN, and the p-type group III nitride semi contractor layer made of Mg doped GaN were epitaxially grown sequentially and moreover, after the n-type electrode layer, the translucent electrode, the p-type electrode layer, and the protective film were formed, a wafer was separated by scribing and an LED element was fabricated.

By using the sputtering method, the AlN film as the buffer layer 602 was formed on the α-Al₂O₃ (0001) substrate under the same conditions as those of the first example. After that, the wafer was introduced into the MOCVD device, and the group III nitride semiconductor intermediate layer 603 made of undoped GaN having a film thickness of 5 μm and the n-type group III nitride semiconductor layer 604 made of Si doped GaN having a film thickness of 2 μm were formed. Moreover, the group III nitride semiconductor active layer 605 having the MQW structure which is a lamination structure starting with GaN and ending with GaN and having 5 layers of InGaN with a film thickness of 3 nm and 6 layers of GaN with a film thickness of 16 nm being alternately laminated and the p-type group III nitride semiconductor layer 606 made of Mg Doped GaN having a film thickness of 200 nm were formed.

For the obtained epitaxial wafer, by using the lithography technology and the RIE technology, the translucent electrode 607, the p-type bonding pad electrode 609, the n-type electrode 608, and the protective film 610 were formed as illustrated in FIG. 6. In the third example, ITO (Indium-Tin-Oxide) was used as the translucent electrode, a structure in which Titanium (Ti), Al, and gold (Au) were laminated as the p-type bonding pad electrode, a structure in which nickel (Ni), Al, Ti, and Au were laminated as the n-type electrode, and SiO₂ as the protective film.

The water on which the LED structure obtained as above was formed was separated by scribing into an LED chip of 350 μm square, and this LED chip was placed on a lead frame and was connected to the lead frame by a gold wire so as to have an LED element.

When a forward current was made to flow through the p-type bonding pad electrode and the n-type electrode of the obtained LED element, favorable emission characteristics of a forward voltage at a current of 20 mA of 3.0 V, an emission wavelength of 470 nm, and an emission output of 15 mW were indicated. Such characteristics were obtained without variation for the LED elements fabricated from substantially all the surfaces of the fabricated wafer.

As described above, by using, as the buffer layer 602, the AlN film controlled to the +c polarity, fabricated by using the film-forming method of the group III nitride semiconductor thin film according to the present invention, an LED element having favorable emission characteristics could be obtained. In the third example, the group III nitride semiconductor intermediate layer 603 made of undoped GaN, the n-type group III nitride semiconductor layer 604 made of Si doped GaN, the group III nitride semiconductor active layer 605 having the MQW structure of InGaN and GaN, and the p-type group III nitride semiconductor layer 606 made of Mg Doped GaN were formed by the MOCVD method, but it was confirmed that the similar results could be obtained by fabricating these layers by using the sputtering method. Moreover, when the experiment similar to the third example was repeated several times, it was confirmed that reproducibility was favorable.

First Comparative Example

As a first comparative example of the present invention, an example in which the AlN film was formed on the α-Al₂O₃ (0001) substrate by using the sputtering method without applying the high-frequency bias power to the bias electrode, which is a feature of the present invention, will be described. In the first comparative example, the AlN film was formed by using the same sputtering apparatus 1, substrate holder 111, and film-forming conditions as those in the first example except that the high-frequency bias power is not applied to the bias electrode 103. Moreover, the frequency of the high-frequency power to be applied to the target electrode 102 is set to 13.56 MHz.

For the AlN film fabricated in the first comparative example, the XRD measurement in the 2θ/ω scan mode at a symmetric reflection position, the XRC measurement in the ω scan mode for the AlN (0002) plane (when an analyzer crystal is inserted in the detector and in an open detector state), and the XRC measurement in the φ scan mode for AlN {10-10} plane were made, and it was known that an epitaxial film with c-axis orientation was obtained similarly to the AlN film obtained in the first example, and mosaic expansion of tilt and twist was at the same degree. On the other hand, when the CAICISS measurement was made for the AlN film fabricated in the first comparative example, it was indicated that +c polarity and −c polarity were mixed in the film.

As described above, if a film was formed without applying high-frequency bias power to the bias electrode 103, it was made clear that the group III nitride semiconductor thin film with +c polarity could not be obtained. The experiment similar to this comparative example was repeated several times, but the AlN film with +c polarity could not be obtained.

Second Comparative Example

Subsequently, as a second comparative example of the present invention, an example in which a buffer layer made of AlN was formed on the α-Al₂O₃ (0001) substrate by using the sputtering method without applying the high-frequency bias power to the bias electrode 103 and an undoped GaN film was formed on the buffer layer by using the MOCVD method will be described. In the second comparative example, the buffer layer made of AlN was formed by using the same sputtering apparatus 1, substrate holder 111, and film-forming conditions as those in the first comparative example, and the undoped GaN film was formed under the conditions similar to those in the second example.

A film of a buffer layer made of AlN was formed on the α-Al₂O₃ (0001) substrate by using the sputtering method using the same sputtering apparatus 1, substrate holder 111, and film-forming conditions as those in the first comparative example, and after that, the wafer was introduced into the MOCVD device, and the undoped GaN film having a film thickness of 5 μm was formed.

The surface of the obtained undoped GaN film was cloudy, and it was indicated that in the XRD measurement in the 2θ/ω scan mode at a symmetric reflection position, the undoped GaN film had c-axis orientation. Subsequently, the XRC measurement in the ω scan mode using the GaN (0002) plane as a symmetric plane and the XRC measurement, in the φ scan mode for GaN {10-10} plane in the In-plane arrangement were made, and it was confirmed that the FWHMs were approximately 600 arcsec and 1000 arcsec, respectively. From this fact, it was known that the undoped GaN film obtained by the second comparative example was obtained as a low-quality crystal with mosaic expansion of tilt and twist larger than that of the undoped GaN film obtained in the second example.

Moreover, it was confirmed by the CAICISS measurement that the obtained undoped GaN film was a film in which +c polarity and −c polarity were mixed. This can be considered as the result that, as explained in the first comparative example, since the buffer layer made of AlN was a film in which the +c polarity and the −c polarity were mixed, the undoped GaN film formed on the buffer layer took over the mixed polarity thereof.

As described above, if the buffer layer made of AlN is formed on the α-Al₂O₃ (0001) substrate by using the sputtering method without applying the high-frequency bias power to the bias electrode, the undoped GaN film grown on the buffer layer by using the MOCVD method is obtained as a low-quality epitaxial film. Though the undoped GaN film was formed by the MOCVD method in the second comparative example, it was confirmed that the similar result is obtained, too, even when the sputtering method is used. Moreover, the experiment similar to this comparative example was repeated several times, but a GaN film with a mirror surface and favorable crystalline could not be obtained.

Third Comparative Example

As a third comparative example of the present invention, an example in which a buffer layer made of AlN was formed on the α-Al₂O₃ (0001) substrate by using the sputtering method without applying the high-frequency bias power to the bias electrode and on the buffer layer, the group III nitride semiconductor intermediate layer made of undoped GaN, the n-type group III nitride semiconductor layer made of Si doped GaN, the group III nitride semiconductor active layer having the MQW structure of InGaN and GaN, and the p-type group III nitride semiconductor layer made of Mg Doped GaN were epitaxially grown sequentially by using the MOCVD method and moreover, after the n-type electrode layer, the translucent electrode, the p-type electrode layer, and the protective film were formed, the wafer was separated by scribing so as to fabricate an LED element will be described.

The film-forming method of the buffer layer made of AlN is similar to the first comparative example, and the materials and film-forming method of the group III nitride semiconductor intermediate layer made of undoped GaN, the n-type group III nitride semiconductor layer made of Si doped GaN, the group III nitride semiconductor active layer having the MQW structure of InGaN and GaN, and the p-type group III nitride semiconductor layer made of Mg doped GaN whose films were formed by using the MOCVD method and the n-type electrode layer, the translucent electrode, the p-type electrode layer, and the protective film formed after that and the step of making an element after that are all similar to those in the third example.

When a forward current was made to flow through the p-type bonding pad electrode and the n-type electrode of the obtained LED element, favorable element characteristics could not be obtained such that favorable diode characteristics could not be obtained from the LED element, sufficient emission intensity could not be obtained in a visible light region and the like. Such characteristics were obtained without variation for the LED elements fabricated from substantially all the surfaces of the fabricated wafer.

As described above, it was made clear that an LED element having favorable emission characteristics could not be obtained if a buffer layer made of AlN was formed on the α-Al₂O₃ (0001) substrate by using the sputtering method without applying she high-frequency bias power to the bias electrode. In this example, the group III nitride semiconductor intermediate layer made of undoped GaN, the n-type group III nitride semiconductor layer made of Si doped GaN, the group III nitride semiconductor active layer having the MQW structure of InGaN and GaN, and the p-type group III nitride semiconductor layer made of Mg doped GaN were formed by using the MOCVD method, but it was confirmed that the similar result was obtained, too, even if the sputtering method was used. The experiment similar to this comparative example was repeated several times, but an LED element having favorable emission characteristics could not be obtained.

Fourth Example

As a fourth example of the present invention, an example in which frequencies of the high-frequency power to be applied to the target electrodes 102 and the high-frequency power to be applied to the bias electrode 103 were both set to 13.56 MHz, the phase was shifted by 180°, the apparatus and conditions similar to those in the first example were used for the others, and an AlN film was formed on the α-Al₂O₃ (0001) substrate by using the film-forming method of the group III nitride semiconductor thin film according to the present invention will be described.

The experiment of the fourth example was conducted repeatedly and it was confirmed that an AlN film with +c polarity similar to that in the first example could be obtained with good reproducibility.

Fourth Comparative Example

As a fourth comparative example of the present invention, an example in which frequencies of the high-frequency power to foe applied to the target electrode 102 and the high-frequency power to be applied to the bias electrode 103 were both set to 13.56 MHz, the apparatus and conditions similar to those in the first example were used for the others, and an AlN film was formed on the α-Al₂O₃ (0001) substrate by using the film-forming method of the group III nitride semiconductor thin film according to the present invention will be described. In the fourth comparative example, phases of the high-frequency power to be applied to the target electrode 102 and the high-frequency power to be applied to the bias electrode 103 were not controlled.

The experiment of the fourth comparative example was conducted repeatedly and it was made clear that, if frequency interference did not occur, an AlN film with +c polarity was obtained, but if the frequency interference occurred, it became difficult to obtain the AlN film with +c polarity.

As described above about the present invention, the major feature of the present invention is that attention is paid to application of the high-frequency bias power to the bias electrode in forming an epitaxial film of a group III nitride semiconductor by the sputtering method. The electric field of the sheath region S generated on the film formation surface side of the substrate by means of application of the high-frequency bias power to the bias electrode is made to act on polarization of the nitride molecule emitted from the target so as to control orientation, and by using the orientation, a group III nitride semiconductor thin film with +c polarity can be obtained, which is an unprecedented technical idea.

Moreover, by preventing or reducing low-frequency beat caused by interference between the high-frequency power applied to the target electrode and the high-frequency power applied to the bias electrode, that is, frequency interference, a group III nitride semiconductor thin film with + c polarity can be obtained with good reproducibility, which is an unprecedented technical idea.

In the present invention, under the technical idea specific to the present invention, the heater electrode and the bias electrode are provided on the substrate holder. By configuring the substrate holder as above, as illustrated in the above-described first to fourth examples and the first to fourth comparative examples, a group III nitride semiconductor thin film with reduced mosaic expansion of tilt and twist and having +c polarity can be formed by the sputtering method. 

1. An epitaxial film forming method that uses a sputtering apparatus having a target electrode having a target containing a group III nitride molecule arranged thereon and a substrate holder capable of holding a sapphire substrate toward the target electrode and provided with a heater electrode and a bias electrode, and that epitaxially grows a group III nitride semiconductor thin film by a sputtering method on a sapphire substrate arranged on the substrate holder, the method comprising: arranging the sapphire substrate on the substrate holder; and forming an epitaxial film of the group III nitride semiconductor thin film immediately on the sapphire substrate arranged on the substrate holder by the sputtering method by applying high-frequency power to the target electrode and by applying high-frequency bias power to the bias electrode, wherein in the forming an epitaxial film of the group III nitride semiconductor thin film immediately on the sapphire substrate: the sapphire substrate at a predetermined temperature is maintained by the heater electrode; a group III nitride molecule is emitted from the target arranged on the target electrode by applying the high-frequency power to the target electrode, a group III nitride element included in the group III nitride molecule being selected from Al, Ga, In and combinations thereof; and high-frequency bias power is applied to the bias electrode for generating an electric field in which polarization from a negative charge to a positive charge of the group III nitride molecule is oriented toward the sapphire substrate arranged on the substrate holder, and wherein the high-frequency power and the high-frequency bias power are applied so that frequency interference between the high-frequency power and the high-frequency bias power does not occur.
 2. The epitaxial film forming method according to claim 1, wherein for frequencies of the high-frequency power and the high-frequency bias power, different frequencies are selected.
 3. The epitaxial film forming method according to claim 1, wherein the high-frequency power and the high-frequency bias power, for which the same frequency is selected are applied so that a phase difference becomes 180°.
 4. The epitaxial film forming method according to claim 1, wherein the bias electrode has a first electrode to which a positive DC voltage is applied and a second electrode to which a negative DC voltage is applied, in a state in which the positive DC voltage is applied to the first electrode and the negative DC voltage is applied to the second electrode, the sapphire substrate is electrostatically adsorbed by the substrate holder, and an epitaxial film of the group III nitride semiconductor thin film is formed on the sapphire substrate.
 5. The epitaxial film forming method according to claim 1, wherein the high-frequency bias power is applied after the high-frequency power is applied and before a film formation surface of the sapphire substrate is covered by a crystal layer made of a group III nitride semiconductor.
 6. A manufacturing method of a semiconductor light-emitting element comprising: forming a buffer layer of a semiconductor light-emitting element by the epitaxial film forming method according to claim
 1. 7.-13. (canceled)
 14. The epitaxial film forming method according to claim 1, wherein in the forming the epitaxial film of the group III nitride semiconductor thin film immediately on the sapphire substrate, the epitaxial film of the group III nitride semiconductor thin film with +c polarity is formed immediately on the sapphire substrate. 